1. Field of the Invention
The present invention relates to a scanning optical system and a scanning optical apparatus for focusing a light beam such as a laser beam onto a scanned surface for scanning, and more particularly to a scanning optical system and a scanning optical apparatus suitable as an image writing means used in a digital image forming apparatus such as a digital copier and a printer.
2. Description of the Prior Art
In recent years, an increasing number of digital image forming apparatuses adopt, as the scanning optical systems that they use as image writing means, multiple-beam optical systems to cope with increasingly high-speed, high-resolution, and full-color image formation. Multiple-beam optical systems are optical systems in which the light beams emitted from a plurality of light sources are simultaneously shone onto a scanned surface to form an image thereon. For example, nowadays, the use of a scanning optical system that, by shining the light beams emitted from a plurality of light sources onto a single scanned surface to scan the surface and thereby form an image thereon, achieves higher-speed and higher-resolution image formation is not limited to applications that require tandem-type structures.
Moreover, Japanese Laid-open Patent Application No. H8-313833 proposes, for use in tandem-type structures, a color image forming apparatus in which a plurality of light sources are used separately as light sources for cyan, magenta, yellow, and black colors, and in which image formation is performed through a scanning optical system separately for individual colors and the obtained images are synthesized on a belt-shaped transfer material to form a color image thereon.
Moreover, for example, Japanese Laid-open Patent Application No. H4-149481 proposes a method for reducing variation in magnification and thereby reducing misplacement of colors by measuring the magnification in the main scanning direction with a sensor and controlling the modulation of a laser in accordance with measurement results. It is quite common to measure magnification by means of a sensor, and it is also quite common to adjust magnification by varying the modulation of a laser, or alternatively by moving a lens, mirror, or other optical component to vary the focal length or conjugate distance.
However, in the conventional scanning optical systems mentioned above, higher resolution, higher precision in printing positions, and stable spot diameter and shape cannot be obtained without compensating for spot diameter differences between light beams and printing position deviations in the main scanning direction resulting from wavelength variation differences between light sources, or from temperature differences between lenses, i.e. variation in focal length. This can be achieved by the use of expensive achromatic lenses, or by the use of a temperature-compensated optical system. However, in these conventional scanning optical systems, compensation for such variation is possible only to a limited degree because the scanning lens has a strong optical power in the main scanning direction.
One way to overcome this problem is to monitor the variation of the printing position and of the image-plane position and move lenses or change beam directions in accordance with monitored results. This, however, requires high-precision detecting and driving systems that tend to be very expensive, and even such systems only offer a limited degree of precision, a limited number of steps, and a limited extent of compensation. On the other hand, the method proposed in Japanese Laid-open Patent Application No. H4-149481 mentioned above requires expensive measurement and feedback means, and in addition requires a larger printer head so that a moving mechanisms and a sensor can be arranged therein.